Conventionally, there is known a brake device including a boost device (a booster) arranged to generate an assist force for decreasing a brake operation force of a driver by an energy source which is different from the brake operation force of the driver. For example, a brake device described in a patent document 1 includes a hydraulic pressure booster serving as a booster device. The hydraulic pressure booster is arranged to increase amplify the depression force of the brake pedal by using a pressure energy stored in an accumulator by a pump, and to transmit it to a master cylinder.